Circuit boards that carry electronic integrated circuits and discrete electronic components are well known. A circuit board substrate is prepared with predetermined conductor paths and pads for receiving the leads of electronic components such as integrated circuit chips, resistors or capacitors. During the circuit board assembly process, solder paste deposits are placed onto the board substrate at appropriate positions. The solder paste deposits are usually applied by placing a stencil screen onto the substrate, applying solder paste through the stencil openings and removing the stencil from the substrate. The circuit board electronic components are then positioned onto the substrate, preferably with a pick and place machine, with leads of the electronic components placed on the respective solder paste deposits. The circuit board is passed through an oven after all of the components are positioned on the substrate to melt the solder paste deposits thus creating an electrical as well as mechanical connection between the components and the substrate.
The size of the solder paste deposits and electronic components and the accuracy with which they must be placed on the substrate has become increasingly smaller and tighter with the increased emphasis on miniaturization in the electronics industry. Solder paste deposit heights can be as small as 50 microns and the height of the solder paste brick must often be measured to within 1 percent of the designed height and size. The center-to-center spacing between solder bricks is sometimes as little as 200 microns. Too little solder paste can result in no electrical connection between the lead of an electronic component and the pad of the circuit board substrate. Too much paste can result in bridging and short-circuiting between the leads of a component. Discrete electronic components such as resistors and capacitors can be as small as 200×400 microns and leads on micro ball grid array components can have a center-to-center spacing less than 300 microns.
A single circuit board can cost thousands and even tens of thousands of dollars to manufacture. Testing of a circuit board after the fabrication process is complete can detect errors in solder paste placement and component placement and lead connection, but often the only remedy for a faulty board is rejection of the entire board. In addition, with the miniaturization of components, visual inspection of the circuit board, even with optical magnification, is unreliable. It is accordingly imperative that a circuit board be inspected during the fabrication process so that improper solder paste placement can be detected prior to the placement of the electronic components onto the substrate. Such in-process solder inspection reduces the cost of failure since expensive components have not yet been placed onto the circuit board.
To ensure proper operation of a solder paste inspection system, the system must be programmed or otherwise taught where on the circuit board the solder paste is to be printed and the expected shape and size of each of the solder paste deposits. This information is required by the inspection system so that measured solder paste deposits' position, height, shape and volume can be compared with expected target values. In previous techniques for programming solder paste inspection systems, computer-aided-design (CAD) files were used and converted into a suitable file format usable by the inspection system. The CAD files used to determine the position and shape of the solder paste deposit are typically in a format called GERBER which is a plotter command based description of the stencil apertures. Typically, conversion of the GERBER file to a solder paste inspection program requires a sophisticated conversion software package which requires trained personnel to operate. In addition, the CAD files that describe the stencil design are occasionally not available to operators setting up a circuit board assembly line which makes it impossible to set up the solder paste inspection system for that circuit board assembly.
Also, there may be an error the fabrication of the stencil or the operator may want to verify the stencil in question matches the CAD data used to fabricate the circuit board.
During the solder paste printing process, a squeegee is drawn across the stencil and solder paste is forced through the stencil apertures to form the solder paste deposit on the circuit board. The process of drawing the squeegee across the stencil stretches and deforms the stencil. Also, repeated use of the stencil can produce dents and deformities in the stencil. The stretching and coining of the stencil over time reduces the quality of the solder printing process. Currently, there is no effective method of determining if a stencil needs to be replaced due to this type of wear.